New tool effectively identifies both rapid-onset and sustained droughts

Photo credit: USDA/NRCS

July 20, 2016

A new experimental drought warning product has the potential to become a leading indicator of
both rapidly evolving 'flash' droughts (developing over a few weeks) and sustained droughts
(developing over months but lasting up to years) for agricultural and hydrological communities.
The Evaporative Demand Drought Index (EDDI) has also proven to be just as good—if not
better—than other drought indices at providing early drought warnings.

What sets EDDI apart is that it takes into account the strong physical relationship between two
factors: 1) evaporative demand—the "thirst of the atmosphere," estimated to be the amount of
water that would evaporate from the soil and be given off as water vapor (transpired) by plants
if the soil were well watered—and, 2) the actual loss of available moisture from the land
surface through evapotranspiration—the sum of plant transpiration and evaporation from the
Earth's land and ocean surface to the atmosphere.

EDDI measures the signal of drought by using daily atmospheric conditions and estimating their
impact on land-surface moisture and vice versa.

"The idea for EDDI occurred while I was making maps of evaporative demand," said
Mike Hobbins, a CIRES researcher with NOAA ESRL’s Physical Sciences Division. "I noticed that with
that information, I could pick out the droughts."

In a pair of complementary articles published in the June issue of the Journal of
Hydrometeorology, Hobbins and his colleagues first present the physical basis and utility of
EDDI. Next, they present a study of its use at multiple timescales for several hydroclimates
across the continental United States, and compare it to other commonly used drought indices—the
Standardized Precipitation Index (SPI), the Standardized Soil Moisture Index (SSI), and the
Evaporative Stress Index (ESI)—and the U.S. Drought Monitor (USDM), which combines numerous
drought indicators with local expert knowledge to provide a national drought map.

Development of a flash drought in the Midwest in 2012. The 2-week EDDI (right) is compared at 5-week intervals to the US Drought Monitor (left). EDDI captures the severe drought condition two months ahead of the USDM. (Credit: Mike Hobbins, CIRES–NOAA)

Development of a flash drought in the Midwest in 2012. The 2-week EDDI (right) is compared at 5-week intervals to the US Drought Monitor (left). EDDI captures the severe drought condition two months ahead of the USDM. (Credit: Mike Hobbins, CIRES–NOAA)

Most drought monitoring tools focus solely on the supply side of moisture imbalance, using
climate variables such as precipitation, soil moisture, and air temperature. EDDI uses air
temperature data, but not precipitation and soil moisture—which are not always available or
reliable. In addition, it incorporates other climatic factors, including solar radiation, winds,
and humidity, which are also important drivers in the depletion of soil moisture and in the
evolution and persistence of drought. From this atmospheric data, Hobbins creates a historical
dataset from 1980 onward across the continental US. This is then used to compare incoming data
that reflect current or ongoing conditions.

EDDI's effectiveness in reflecting the moisture conditions on the land surface is based on
feedbacks between the atmosphere and land that are particularly strong during the warm season,
when drought is of greatest concern. "The dryness at the surface affects the dryness of the
air," said Hobbins. "There is a quick feedback."

Another advantage of EDDI is that its calculation is not sensitive to land-surface type, so it
is a valid drought indicator for all regions.

So how did EDDI compare to the other drought indicators in the study? Overall, EDDI identified
drought consistently with most of the other indices. For flash droughts in particular, EDDI
predicted the development and onset of drought up to two months ahead of the USDM, and often led
both the SPI and the SSI. And although it works without precipitation information, EDDI was
still able to capture long-term hydrologic and snow drought in the Western U.S.

"These results are promising," said Hobbins. "They allow us to think that we are approaching the
brass ring of drought monitoring—effective prediction of drought—but we need to examine EDDI's
performance in an ongoing monitoring setting, which is what is being done now in concert with
our stakeholders across the continental US."

Hobbins is currently responsible for generating and distributing the drought warning tool to
stakeholders who have requested the information for their region. However, EDDI will soon become
available online through drought.gov, and plans are in the works for it to be permanently hosted
by NOAA's National Water Center within the next few years.

What's next?

A number of improvements are ongoing or planned for EDDI, such as testing the tool
by feedback from early users and then changing the system in response; expanding the user-base
to a national basis; examining EDDI as a predictor of wildfire risk; attributing changes in
evaporative demand (or of drought) to changes on each of the drivers (wind, air temperature,
humidity, solar radiation); working with the Famine Early Warning System Network to expand the
tool from the continental US to a global basis; and seasonal forecasting of the evaporative
demand that underlies EDDI, which could potentially allow forecasts of drought.